1. Field of the Invention
The present invention relates to an image forming apparatus which forms a visible image by developing an elestrostatic latent image.
2. Description of the Related Art
In a conventional image forming apparatus, first and second developer bearing members are disposed along a moving path of an image bearing member, and each of the developer bearing members supplies the image bearing member with a developer having a same color.
FIG. 1 illustrates a principal portion of the above-described image forming apparatus. A cylindrical electrophotographic photosensitive member, or photosensitive drum 1, serves as the image bearing member, and rotates in a clockwise direction during an image forming operation. An electrostatic latent image formed on photosensitive drum 1 is developed by developing unit 10. Developing unit 10 includes receptacle or container 11 having a chamber 12 for containing a developer.
Cylindrical developer bearing members, i.e., developing rollers 2 and 5, are rotatably supported in receptacle 11. First and second developing rollers 2 and 5 rotate in a counterclockwise direction during an image forming operation, to carry the developer contained within developer accommodating chamber 12 and to supply the developer to photosensitive drum 1.
As shown in FIG. 1, first and second developing rollers 2 and 5 are disposed in parallel with an interval spacing in the direction of rotation of drum 1. Each of first and second developing rollers 2 and 5 carries the developer having the same color, i.e., the same component, and supplies the developer to photosensitive drum 1. Accordingly, the electrostatic latent image formed on photosensitive drum 1 is first subjected to preliminary development by first developing roller 2, and then subjected to finishing development by second developing roller 8.
As shown in FIG. 2, spacer rollers 3 and 4 are rotatably supported on shafts 2a and 2b present at two end-portions of first developing roller 2 in the longitudinal direction, respectively, and spacer rollers 6 and 7 are rotatably supported on shafts 5a and 5b present at two end-portions of second developing roller 5 in the longitudinal direction, respectively. That is, spacer rollers 3 and 4, and spacer rollers 6 and 7 are provided coaxially with respect to first and second developing rollers 2 and 5, respectively.
As shown in FIG. 1, the above-described spacer rollers 3 and 4, and spacer rollers 6 and 7 are in pressure contact with two end-portions of photosensitive drum 1 in the longitudinal direction. Thus, the gap between photosensitive drum 1 and first developing roller 2, and the gap between photosensitive drum 1 and second developing roller 5 are maintained constant.
Developing unit 10 is supported on guide member 13, and can be longitudinally advanced and retracted with respect to photosensitive drum 1. More specifically, foot member 17 protruding from receptacle 11 of developing unit 10 is slidably supported on guide member 13.
As will be described later, developing unit 10 can be detached from the main body of the image forming apparatus by drawing developing unit 10 along guide 13 forwardly with respect to the plane of FIG. 1 in a state in which developing unit 10 is retracted from photosensitive drum 1. On the other hand, developing unit 10 can be mounted on the main body of the image forming apparatus by inserting developing unit 10 along guide 13 backwardly with respect to the plane of FIG. 1 while mounting foot member 17 of developing unit 10 on guide 13.
Guide member 13 is supported by guide member 16 fixed to the main body of the image forming apparatus so that it can be advanced and retracted with respect to photosensitive drum 1. Two pressure-receiving members 13a and 13b facing each other are provided on guide member 13. The two pressure-receiving members 13a and 13b engage eccentric cam 15 fixed to shaft 14.
That is, when shaft 14 is rotated by a motor or manually in a counterclockwise direction, pressure receiving member 13a of guide member 13 is pushed by cam 15, whereby developing unit 10 moves together with guide member 13 in the direction of arrow A shown in FIG. 1. In this manner, spacer rollers 3, 4, 6 and 7 of developing unit 10 are disposed in pressure contact with photosensitive drum 1.
When developing unit 10 or photosensitive drum 1 is to be detached from the image forming apparatus, shaft 14 is rotated by a motor or manually in a clockwise direction, whereby pressure-receiving member 13b of guide member 13 is pushed by cam 15, and developing unit 10 moves together with guide member 13 in the direction of arrow B shown in FIG. 1. In this manner, developing unit 10 is removed from pressure contact with photosensitive drum 1. That is, spacer rollers 3, 4, 6 and 7 are removed from pressure contact with photosensitive drum 1.
In FIG. 2, gear 8 is fixed to shaft 2b which is fixed to first developing roller 2, and gear 9 is fixed to shaft 5b which is fixed to second developing roller 5. Idle gear 10 meshes gears 8 and 9. When developing unit 10 is in pressure contact with photosensitive drum 1 as shown in FIG. 1, gear 9 meshes with driving gear 18 provided in the main body of the image forming apparatus (not shown). Gear 18 is rotatably driven by a motor (not shown), whereby developing rollers 2 and 5 are rotatably driven in the above-described manner.
The mutual positional relationship among the four spacers 3, 4, 6 and 7 is fixed in developing unit 10 shown in FIGS. 1 and 2.
Accordingly, for example, if the mutual positional relationship among the four spacers 3, 4, 6 and 7 deviates from a predetermined relationship due to error in production, or if tolerances in guide member 13 or 16 are inferior, a case may arise in which only three of the four spacers are disposed in pressure contact with photosensitive drum 1.
In such a case, the gap between the first developing roller 2 and the photosensitive drum 1, or the gap between the second developing roller 5 and the photosensitive drum 1 is not constant with respect to the longitudinal direction of the corresponding developing roller, thereby causing a variation in the picture quality of the formed developed image in the direction of the width.
Such a problem can be overcome in an apparatus shown in FIG. 3. In the apparatus of FIG. 3, first developing unit 101 is configured by providing first developing roller 2 in receptacle 11, and second developing unit 102 is configured by providing second developing roller 5 in receptacle 11'. Spacer rollers 3 and 4 are provided at two end-portions of roller 2, and spacer rollers 6 and 7 are provided at two end-portions of roller 5.
First developing unit 101 and second developing unit 102 are advanced and retracted, thereby to pressure contact photosensitive drum 1 by cams 15 and 15', respectively. Thus, developing units 101 and 102 can be pressed against photosensitive drum 1 so that developing rollers 2 and 5 are parallel to photosensitive drum 1.
In this apparatus, however, two pressing means (15 and 14, and 15' and 14') are required, thereby requiring a complicated and large apparatus. Furthermore, it is necessary to perform operations for individually mounting or detaching the two developing units 101 and 102 with respect to the image forming apparatus, thereby causing an increase in the burden of the operator.
In FIG. 3, components represented by numerals having apostrophes have the same functions as components represented by the same numerals not having apostrophes.